By Sean Westerveld and Melanie Filotas, OMAFRA
Over the last few weeks we have had a number of questions about “mildew”. Mildew can refer to either downy or powdery mildew, and it seems that some growers have been confusing the two. While both cause fuzzy growth on plant leaves, that is where the similarities end. Downy and powdery mildew are actually very different diseases with different management strategies, and mistaking the two can be costly.
Downy and powdery mildew are common to a wide variety of fruit, vegetable and even some field crops. They tend to be relatively specific, attacking only one or a few closely related crops. So, for example, the downy mildew of basil will not affect cucurbits, and vice versa. Some crops are affected by both a powdery mildew and a downy mildew disease (e.g. hops and cucurbits).
Although downy mildews are often thought of as fungal diseases, the pathogens involved are actually water moulds, and more closely related to certain algae. Downy mildews can only infect if free water is present on leaf surfaces, which is why these diseases often become worse with rainy weather and, in some crops, do not occur during dry years. Infection in most crops leads to angular, yellowish or olive green lesions on the upper surface of leaves (Figure 1 and 2), and white, grey, purple or dark sporulation on the lower leaf surface (Figure 3). This sporulation is not always present, and is most easily observed when leaf surfaces are wet or in the early morning before the spores are blown off of the plant. After 1 to 3 weeks affected leaves will die or fall off of the plant. Downy mildews are often more devastating than powdery mildews, leading to complete canopy destruction if not controlled preventatively. They are much harder to control than powdery mildews.
In contrast, powdery mildew diseases are caused by various species of fungi. Spread of powdery mildew is favoured by moderate temperatures and high humidity. Free water is not required for infection, so spread of the disease can occur even without rainy weather. The disease often begins in late summer when the foliage is already weakened and humidity is higher around the leaves within the dense canopy. Powdery mildew causes a white or grey powdery fungal growth, which can occur anywhere on the leaf but is more common on the upper leaf surface (Figure 4). The fungus grows predominantly on the leaf surface. Symptoms can start as roughly circular patches of white growth, but later the entire leaf surface becomes uniformly covered with the fungus. After several weeks the leaves continue to weaken and eventually turn yellow and die, but the progress is usually slower than downy mildew. Powdery mildew species both grow and sporulate on the leaf surface while downy mildew grows inside the leaf tissue, with only the spores produced on the leaf surface.
Most importantly, growers need to be aware that downy and powdery mildew are controlled by very different chemistries. There are very few products that have efficacy against both diseases. For example, some of the commonly used downy mildew protectant fungicides such as Revus and Zampro will have no effect on powdery mildew. The products that have some efficacy against both diseases (e.g. chlorothalonil and mancozeb products) rarely provide sufficient control of downy mildew under high disease pressure. Because the powdery mildew pathogen lives on the surface of the leaf, it is easier for contact fungicides to reach and control the disease. Downy mildew control requires products that can penetrate the leaf surface, or are put on the leaf preventatively as a barrier to prevent spores from germinating. Since downy mildews can quickly destroy a canopy, it is very important to control these diseases preventatively, and products need to be applied before symptoms occur in the field when conditions are favourable for disease development.
Control of these diseases in organic production can be very challenging. There are relatively few organically acceptable fungicides registered and even fewer will have efficacy against both diseases. Prior to application of any product, growers should ensure that products used are registered for use in Canada by the PMRA and are permitted by their certification body.
There are more organically-acceptable products registered for use on powdery mildew than for downy mildew, although most will provide suppression rather than control. Most of these products will have no effect on downy mildew. Homemade pesticides are also more likely to be effective against powdery mildew, as almost all popular recipes will only kill on contact and powdery mildew grows on the leaf surface where it can be more easily contacted. They will not have much effect on downy mildew, since the pathogen grows within the leaf where the product cannot reach them. Copper is one example of an organically-acceptable pesticide that can control or suppress both diseases. However excessive use of copper can cause damage to the soil microflora and in some crops ( e.g. cucurbits) copper can even be phytotoxic. Additionally, downy and powdery mildew are caused by different species on different crops, so management strategies that work on one crop may not work on another.
Other than copper, very few organically-acceptable pesticides provide any control of downy mildew. Research at the University of Guelph on a range of products on basil downy mildew over a three-year period did not identify any organically-acceptable pesticides that could provide more than a very slight reduction in the severity of the disease.
The main tool for organically managing both downy and powdery mildew, and an essential part of disease management for all growers, is reducing or avoiding the conditions that lead to early disease or severe infections. This can be accomplished by choosing cultivars that are less susceptible to the disease most prevalent or damaging on your crop and farm, if available, ensuring wide plant spacing to avoid a dense canopy with limited air flow, avoiding overhead irrigation, and planting in an open location exposed to the prevailing winds. In some cases, removing and destroying the first infected leaves or stems will reduce inoculum and delay the onset of severe symptoms, but this requires frequent and careful scouting.